Perfluoroelastomer composition having improved processability

ABSTRACT

Perfluoroelastomer compositions of improved processability are provided which have reduced levels of ionized or ionizable polymer endgroups.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.08/909,898, filed Aug. 12, 1997 U.S. Pat. No. 5,789,509 which is acontinuation of U.S. application Ser. No.08/755,946, filed Nov. 25,1996, abandoned.

FIELD OF THE INVENTION

This invention relates to perfluoroelastomer compositions which haveexcellent processability, and which, when cured, have outstandingthermal stability and chemical resistance.

BACKGROUND OF THE INVENTION

Perfluoroelastomers (elastomeric perfluoropolymers) are polymericmaterials which exhibit outstanding high temperature tolerance andchemical resistance. Consequently, such compositions are particularlyadapted for use as seals and gaskets in systems in which elevatedtemperatures and/or corrosive chemicals are encountered. The outstandingproperties of perfluoropolymers are largely attributable to thestability and inertness of the copolymerized perfluorinated monomerunits which make up the major portion of the polymer backbone, e.g.,tetrafluoroethylene and perfluoro(alkyl vinyl) ethers. In order tocompletely develop elastomeric properties, perfluoropolymers aretypically crosslinked, i.e. vulcanized. To this end, a small percentageof cure site monomer is copolymerized with the perfluorinated monomerunits. Cure site monomers containing at least one nitrile group, forexample perfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene, are especiallypreferred. Such compositions are described in U.S. Pat. Nos. 4,281,092and 4,394,489; and in International Application WO 95/22575.

A recently-developed class of perfluoroelastomers havingcarbonyl-containing functional groups is disclosed in co-pending U.S.patent application Ser. No. 08/755,919, entitled "Fast-curingPerfluoroelastomer Compositions", filed Nov. 25, 1996. These polymersare characterized by having carbonyl-containing functional groups,including carboxyl groups, carboxylate groups, carboxarnide groups, andmixtures thereof. Preferably, the carbonyl-containing functional groupsare endgroups generated as a result of persulfate initiation of thepolymerization reaction and the reaction is carried out in the absenceof sulfite or bisulfite reducing agents. The carbonyl-containingperfluoroelastomers exhibit outstanding cure characteristics but theyare difficult to process in certain end-uses because of their relativelyhigh viscosity. A method for decreasing viscosity of thecarbonyl-containing perfluoroelastomers would permit use of thesematerials in a wider variety of end-use applications.

SUMMARY OF THE INVENTION

The present invention provides a composition comprising aperfluoroelastomer comprising copolymerized units of (1) aperfluoroolefin, (2) a perfluorovinyl ether selected from the groupconsisting of perfluoro(alkyl vinyl) ethers, perfluoro(alkoxy vinyl)ethers, and mixtures thereof, and 3) a cure site monomer wherein theperfluoroelastomer is characterized by being substantially free offunctional groups selected from the group consisting of i) ionized orionizable endgroups, ii) bromine-containing endgroups, and iii)iodine-containing endgroups.

The invention is further directed to a curable perfluoroelastomercomposition comprising

A) a perfluoroelastomer comprising copolymerized units of (1) aperfluoroolefin, (2) a perfluorovinyl ether selected from the groupconsisting of perfluoro(alkyl vinyl) ethers, perfluoro(alkoxy vinyl)ethers and mixtures thereof, and 3) a cure site monomer, wherein theperfluoroelastomer is characterized by being substantially free offunctional groups selected from the group consisting of i) ionized orionizable endgroups, ii) bromine-containing endgroups, and iii)iodine-containing endgroups; and

B) a curing agent.

The invention is also directed to a process for preparation of anuncured perfluoroelastomer composition comprising the steps of

A) preparing a perfluoroelastomer having a plurality ofcarbonyl-containing functional groups selected from the group consistingof carboxyl endgroups, carboxylate endgroups, carboxamide endgroups, andmixtures thereof by copolymerizing a monomer mixture comprising a) aperfluoroolefin monomer, b) a perfluorovinyl ether monomer selected fromthe group consisting of perfluoro(alkyl vinyl) ethers, perfluoro(alkoxyvinyl) ethers, and mixtures thereof, and c) a cure site monomer at apressure of from 4-10 MPa, in the presence of a persulfate free radicalinitiator, in a polymerization mixture wherein i) the feed ratio ofmonomer to initiator is controlled so that the ratio of the radical fluxto the polymerization rate, R_(i) /R_(p), is from about 10 to 50millimoles per kilogram, and ii) less than 5 mole percent of a sulfiteor bisulfite reducing agent, based on the total moles of persulfateinitiator and reducing agent, is present in the polymerization mixture;

B) isolating said perfluoroelastomer having a plurality ofcarbonyl-containing functional groups from the polymerization mixture;and

C) heating said isolated perfluoroelastomer having a plurality ofcarbonyl-containing functional groups at a temperature of at least 230°C. for a time sufficient to at least partially decarboxylate theperfluoroelastomer.

In addition, the present invention is directed to an uncuredperfluoroelastomer composition prepared by a process comprising thesteps of

A) preparing a perfluoroelastomer having a plurality ofcarbonyl-containing functional groups selected from the group consistingof carboxyl endgroups, carboxylate endgroups, carboxamide endgroups, andmixtures thereof by copolymerizing a monomer mixture comprising a) aperfluoroolefin monomer, b) a perfluorovinyl ether monomer selected fromthe group consisting of perfluoro(alkyl vinyl) ethers, perfluoro(alkoxyvinyl) ethers, and mixtures thereof, and c) a cure site monomer at apressure of from 4-10 MPa, in the presence of a persulfate free radicalinitiator in a polymerization mixture wherein i) the feed ratio ofmonomer to initiator is controlled so that the ratio of the radical fluxto the polymerization rate, R_(i) /R_(p), is from about 10 to 50millimoles per kilogram, and ii) less than 5 mole percent of a sulfiteor bisulfite reducing agent, based on the millimoles of persulfateinitiator and reducing agent, is present in the polymerization mixture;

B) isolating said perfluoroelastomer having a plurality ofcarbonyl-containing functional groups from the polymerization mixture;and

C) heating said isolated perfluoroelastomer having a plurality ofcarbonyl-containing functional groups at a temperature of at least 230°C. for a time sufficient to at least partially decarboxylate theperfluoroelastomer.

The invention is further directed to curable compositions comprising theproduct produced by the above-described process and a curing agent.

DETAILED DESCRIPTION OF THE INVENTION

The compositions of the present invention comprise perfluoroelastomersof several types which can be classified according to the nature ofpolymer endgroups present. In particular, the perfluoroelastomercompositions are characterized by having endgroups which are non-ionizedor non-ionizable. Perfluoroelastomers having non-ionized ornon-ionizable brominated or iodinated endgroups are known. Theperfluoroelastomers of the present invention are, however, substantiallyfree of brominated or iodinated endgroups.

The first type of perfluoroelastomer of the present invention comprisesperfluoroelastomers which are substantially free of endgroups selectedfrom the group consisting of a) ionized or ionizable endgroups, b)bromine-containing endgroups, and c) iodine-containing endgroups. Withregard to this first type of perfluoroelastomer composition,substantially free of ionized or ionizable endgroups means that lessthan 5% of the polymer endgroups present are ionized or ionizable. Byionized or ionizable endgroups is meant acid endgroups and endgroupswhich are salts of acids. Examples of ionized or ionizable endgroupsinclude carboxylic acid endgroups, carboxylate endgroups, sulfonic acidendgroups, and sulfonate endgroups. By substantially free ofbromine-containing endgroups and iodine-containing endgroups is meantthat less than 0.01 weight percent iodine or bromine is present in thepolymer endgroups.

A second type of perfluoroelastomer of the present invention comprises aclass of perfluoroelastomers wherein some ionized or ionizablecarbonyl-containing endgroups are present. By ionized or ionizablecarbonyl-containing endgroups is meant carboxylate endgroups orcarboxylic acid endgroups, respectively. Preferably, no more than 80% ofthe endgroups will be represented by ionized or ionizablecarbonyl-containing endgroups because higher levels of such endgroupsare detrimental to polymer processability. However, under high shear,even a 10% reduction of carbonyl-containing endgroups will result inimproved polymer rheology. These compositions are further characterizedin that substantially no type of ionized or ionizable endgroup otherthan carbonyl-containing ionized or ionizable endgroups is present. Bysubstantially no other type of ionized or ionizable endgroup it is meantthat no more than 0.75 millimoles per kilogram of polymer of these otherionizable or ionized endgroups are present. Such other ionized orionizable endgroups include sulfonic acid and sulfonate endgroups. Ifthese non-carboxyl or non-carboxylate groups are present in significantquantity, then the viscosity of the polymer begins to increase, whichmakes polymer processing difficult. Members of this second class ofperfluoroelastomer compositions of the present invention are prepared bypartial decarboxylation of perfluoroelastomers having ionized orionizable carbonyl-containing functional groups.

The present invention also includes curable compositions comprising theabove-described two types of perfluoroelastomers in combination withcuratives.

The present invention is also directed to a process for preparation ofperfluoroelastomers having improved processability. The process involvesreduction of the level of ionized or ionizable carbonyl-containinggroups in the polymer by decarboxylation of perfluoroelastomers havingcarboxyl or carboxylate endgroups or carboxyl or carboxylate pendantfunctional groups.

Perfluoroelastomers are polymeric compositions having copolymerizedunits of at least two principal perfluorinated monomers. Generally, oneof the principal comonomers is a perfluoroolefin while the other is aperfluorovinyl ether. Representative perfluorinated olefins includetetrafluoroethylene and hexafluoropropylene. Suitable perfluorinatedvinyl ethers are those of the formula

    CF.sub.2 ═CFO(R.sub.f' O).sub.n (R.sub.f" O).sub.m R.sub.f(I)

where R_(f') and R_(f") are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R_(f) is a perfluoroalkyl group of 1-6 carbon atoms.

A preferred class of perfluoro(alkyl vinyl) ethers includes compositionsof the formula

    CF.sub.2 ═CFO(CF.sub.2 CFXO).sub.n R.sub.f             (II)

where X is F or CF₃, n is 0-5, and R_(f) is a perfluoroalkyl group of1-6 carbon atoms.

Most preferred perfluoro(alkyl vinyl) ethers are those wherein n is 0 or1 and R_(f) contains 1-3 carbon atoms. Examples of such perfluorinatedethers include perfluoro(methyl vinyl) ether and perfluoro(propyl vinyl)ether. Other useful monomers include compounds of the formula

    CF.sub.2 ═CFO (CF.sub.2).sub.m CF.sub.2 CFZO!.sub.n R.sub.f(III)

where R_(f) is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1,n=0-5, and Z=F or CF₃.

Preferred members of this class are those in which R_(f) is C₃ F₇, m=0,and n=1. Additional perfluoro(alkyl vinyl) ether monomers includecompounds of the formula

    CF.sub.2 ═CFO (CF.sub.2 CFCF.sub.3 O).sub.n (CF.sub.2 CF.sub.2 CF.sub.2 O).sub.m (CF.sub.2).sub.p !C.sub.x F.sub.2x+1             (IV)

where m and n=1-10, p=0-3, and x=1-5.

Preferred members of this class include compounds where n =0-1, m=0-1,and x=1.

Examples of useful perfluoro(alkoxy vinyl) ethers include

    CF.sub.2 ═CFOCF.sub.2 CF(CF.sub.3)O(CF.sub.2 O).sub.m C.sub.n F.sub.2n+1(V)

where n=1-5, m=1-3, and where, preferably, n=1.

Mixtures of perfluoro(alkyl vinyl) ethers and perfluoro(alkoxy vinyl)ethers may also be used.

Preferred copolymers are composed of tetrafluoroethylene and at leastone perfluoro(alkyl vinyl) ether as principal monomer units. In suchcopolymers, the copolymerized perfluorinated ether units constitute fromabout 15-50 mole percent of total monomer units in the polymer.

The perfluoropolymer further contains copolymerized units of at leastone cure site monomer, generally in amounts of from 0.1-5 mole percent.The range is preferably between 0.3-1.5 mole percent. Although more thanone type of cure site monomer may be present, most commonly one curesite monomer is used and it contains at least one nitrile substituentgroup. Suitable cure site monomers include nitrile-containingfluorinated olefins and nitrile-containing fluorinated vinyl ethers.Useful nitrile-containing cure site monomers include those of theformulas shown below.

    CF.sub.2 ═CF--O(CF.sub.2).sub.n --CN                   (VI)

where n=2-12, preferably 2-6;

    CF.sub.2 ═CF--O CF.sub.2 --CFCF.sub.2 --O!.sub.n --CF.sub.2 --CFCF.sub.3 --CN                                         (VII)

where n=0-4, preferably 0-2; and

    CF.sub.2 ═CF-- OCF.sub.2 CFCF.sub.3 !.sub.x --O--(CF.sub.2).sub.n --CN(VIII)

where x=1-2, and n=1-4.

Those of formula (VIII) are preferred. Especially preferred cure sitemonomers are perfluorinated polyethers having a nitrile group and atrifluorovinyl ether group. A most preferred cure site monomer is

    CF.sub.2 ═CFOCF.sub.2 CF(CF.sub.3)OCF.sub.2 CF.sub.2 CN(IX)

i.e. perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) or 8-CNVE.

Other cure site monomers include olefins represented by the formula R₁CH═CR₂ R₃, wherein R₁ and R₂ are independently selected from hydrogenand fluorine and R₃ is independently selected from hydrogen, fluorine,alkyl, and perfluoroalkyl. The perfluoroalkyl group may contain up toabout 12 carbon atoms. However, perfluoroalkyl groups of up to 4 carbonatoms are preferred. In addition, the curesite monomer preferably has nomore than three hydrogen atoms. Examples of such olefins includeethylene, vinylidene fluoride, vinyl fluoride, trifluoroethylene,1-hydropentafluoropropene, and 2-hydropentafluoropropene, as well asbrominated olefins such as 4-bromo-tetrafluorobutene-1 andbromotrifluoroethylene.

An especially preferred perfluoroelastomer contains 53.0-79.9 molepercent tetrafluoroethylene, 20.0-46.9 mole percent perfluoro(methylvinyl) ether and 0.4 to 1.5 mole percent nitrile-containing cure sitemonomer.

Any carbonyl-containing functional groups present in theperfluoroelastomers of this invention are either present as polymerendgroups or as pendant functionalities introduced as a result ofcopolymerization of fluorinated carbonyl-containing comonomers. Forpurposes of this invention carbonyl-containing endgroups are carboxylicacid endgroups, carboxylic acid salt endgroups, or carboxamide (i.e.amides of carboxylic acids) endgroups. By carbonyl-containing comonomeris meant a fluorinated monomer having a copolymerizable double bond andat least one pendant carboxylic acid group (including salts thereof),pendant carboxylic acid ester group, or pendant carboxamide group. Suchcomonomers are represented by compounds represented by formulas (X) and(XI):

    CF.sub.2 ═CFO(CF.sub.2).sub.n X                        (X)

    CF.sub.2 ═CFO CF.sub.2 CF(CF.sub.3)O!.sub.n (CF.sub.2).sub.x X(XI)

where

n=1-4,

x=2-5, and

X=CO₂ H, CO₂ ⁻, CONH₂, or CO₂ CH₃

Depending on the carbonyl-containing comonomer selected, the resultingpolymer would have carboxyl, carboxylate, or carboxamide (i.e.carboxylic acid amide) groups at any point on the chain, i.e. at thechain end, within the chain, or both.

Perfluoroelastomers having carboxyl or carboxylate endgroups can beprepared by polymerization of mixtures of perfluoroolefins,perfluorovinyl ethers, and cure site monomers in the presence of a freeradical generating initiator either in bulk, in solution in an inertsolvent, in aqueous suspension, or in aqueous emulsion.Perfluoroelastomer polymerization techniques are described in general inLogothetis, Prog. Polymn. Sci, Vol. 14, 252-296 (1989) and in co-pendingU.S. patent application Ser. No. 08/755,919 entitled "Fast-curingPerfluoroelastomer Composition," filed Nov. 25, 1996. The Logothetisarticle discloses, among others, a method of polymerization whichinvolves initiation by persulfates, such as ammonium or potassiumpersulfate, in the absence of a reducing agent. Thermally initiatedfree-radical polymerization using persulfates in the absence of areducing agent results in the production of polymers having carboxylicacid end groups which ionize to form carboxylate groups. Reducing agentsinclude such compounds as sodium sulfite and sodium hydrogen sulfite.

As described in co-pending U.S. patent application Ser. No. 08/755,919,carboxylated polymers having superior cure characteristics may beobtained by 1) copolymerizing a mixture of perfluoroolefins,perfluorovinyl ethers, and cure site monomers by initiating thecopolymerization reaction with ammonium persulfate, in the absence of areducing agent, in aqueous emulsion in a continuous well-stirred reactorwith a residence time of 2-4 hours, at a temperature of 75° C.-90° C.and at a pressure of 2-8 MPa. Preferably the residence time is between3.0-3.7 hours, the temperature is 80° C.-85° C., and the pressure is6.0-8.0 MPa. If levels of reducing agent above 5 mole percent (based onthe total number of moles of ammonium persulfate and reducing agent) arepresent, then the amount of sulfonate endgroups on the polymer formedreaches a level which has detrimental effects on polymer processability.In addition, in order to obtain the fast cure rates typical of thecompositions disclosed, the pH of the polymerization reaction mixture isgenerally between 3.5-7.0, preferably between 4.5-6.5.Tetrafluoroethylene and perfluoro(methyl vinyl) ether monomers arepreferred and are fed by compressor. Cure site monomer is preferably fedby liquid metering pump or by a compressor. This polymerization methodresults in production of a perfluoroelastomer copolymer compositionhaving a significant proportion of carboxyl-containing endgroups,carboxylate-containing endgroups, carboxamide endgroups, or mixturesthereof. The number of carboxyl, carboxylate, and carboxamide groupspresent in the nitrile-containing perfluoroelastomers accounts for thecarbonyl content and is related to the ratio of radicals generated topolymerization rate. Specifically, the ratio of the radical generationrate, calculated from persulfate thermal decomposition kinetics, to thepolymerization rate provides a measure of the carbonyl content of thepolymer. Thermal decomposition rates for persulfate are correlated in F.A. Bovey, et al., "Emulsion Polymerization", Interscience Publishers,N.Y., 1955. At 85° C., the first order decomposition rate coefficient is0.011/minute. For a continuous stirred tank reactor at 85° C. and aresidence time of 218 minutes, about 70% of persulfate fed woulddecompose to produce a radical flux R_(i) (mmol/hour) of sulfateradicals equal to 1.4 times the persulfate fed (mmol/hour). Actualinitiator efficiency could be significantly less than that assumed inthis calculation, depending on polymer conditions and type of monomerinvolved. The polymerization rate R_(p) (kg/hour) is readily measured,so that the ratio R_(i) /R_(p) can be calculated for correlation withthe observed carboxylate levels. Generally, for purposes of the presentinvention, the ratio R_(i) /R_(p) should be in the range of about 10-50mmol/kg, preferably 20-40 mmol/kg.

Carbonyl-containing functional groups may also be introduced bycopolymerization of fluorinated carboxyl-containing comonomers with theprincipal perfluoroolefins and perfluorovinyl ether comonomers. Curesite monomers are copolymerized into the polymer as well. Suchcopolymerizations may be conducted substantially as described above. Ifthe sole initiator is a persulfate salt, then carbonyl-containingendgroups will also result. If a sulfite or bisulfite reducing agent isadditionally present, then the resultant copolymers will containsulfonic acid or sulfonate endgroups and carboxyl or carboxylate groups.

The polymer emulsion, upon exiting the reactor, is coagulated with anaqueous solution of a multivalent metal salt, such as magnesium sulfate.The coagulated polymer is then washed with deionized water and dried at70°-100° C. in a circulating air oven.

In order to produce the perfluoroelastomers of the present inventionwhich are substantially free of ionized or ionizable carbonyl-containingendgroups, decarboxylation of carboxylated perfluoroelastomers, such asthose described above, is conveniently carried out by heat-treating thesolid carboxylated perfluoroelastomers, which have been isolated andoven-dried. It is not necessary that the polymer be completely dry. Thatis, the polymer may be completely or partially dried prior to thedecarboxylation process. In order to effect decarboxylation, theperfluoropolymer is heated to a temperature sufficiently high, and for asufficiently long period of time, to decarboxylate all of the endgroupsand convert them to non-ionizable substituents, for example,difluoromethyl groups or trifluorovinyl groups. This results in alowering of polymer viscosity. Partially decarboxylatedperfluoroelastomers are also useful compositions and may be prepared byheat treating the carboxylated perfluoroelastomer for shorter periods oftime. Generally, a temperature of 230° C.-325° C. for a period ofseveral minutes is sufficient to partially decarboxylate the polymer.Thus, a circulating air oven treatment of polymer crumb or sheet attemperatures of about 230°-325° C. is effective in removing a fractionor substantially all of the carbonyl-containing functional groups.Preferably, the polymer will be heated for 30 minutes at a temperatureof 280°-320° C. If the temperature is below 250°, then decarboxylationmay be too slow to be a commercially useful process. However, ifpotassium salts are used in the polymerization mixture, then thedecarboxylation temperature can be lowered to 230° C. without reducingthe rate of decarboxylation to a level which is commerciallyunacceptable. For example, the initiator may be potassium persulfate,and potassium salts may be used as buffers and surfactants. A suitablebuffer is potassium hydrogen phosphate and a suitable surfactant ispotassium perfluorooctanoate. It is desirable to decarboxylate at thelower end of the temperature range to minimize any possibility ofdegradation of copolymerized cure site monomers. If the temperature isbelow 230° C., then decarboxylation is extremely slow. If thetemperature is above 325° C., then the amount of cure site monomer inthe polymer may be reduced by the heat treatment. At the lowesttemperatures the required heating time is much longer than at thehighest temperatures and typical heating times range from about 5minutes to about 24 hours. The decarboxylation can also be performed ina heated extruder, in a compression mold, or any other conventionalheated elastomer processing equipment. The appropriate time will dependon temperature and the degree of decarboxylation desired. It is readilyunderstood by those skilled in the art that other means of increasingthe internal temperature of the polymer may be used, for exampleexposure to microwave radiation.

Unexpectedly, the perfluoroelastomers are not degraded by the heattreating process and retain their excellent response to vulcanizationwith a variety of curing agents. For example, it has been found that ifcopolymerized units of nitrile-containing cure site monomers, e.g.8-CNVE, are present in the perfluoroelastomer, their concentration isessentially unaffected by a properly chosen heating cycle.

The decarboxylation process results in production of aperfluoroelastomer having significantly lowered bulk viscosity comparedto the non-decarboxylated polymer, thus improving processability.Another advantage of the lower bulk viscosity of the decarboxylatedpolymer is that decarboxylated polymers of higher molecular weight thanwould have been processable in the non-decarboxylated form can now beused commercially. These higher molecular weight polymers impartimproved physical properties (e.g. tensile strength, compression set andreduced weight loss at high temperatures) to finished articles. Theviscosity decrease is related to the reduction of ionic difunctionalitythat results from the heat treatment. For example, Mooney viscosity,ML-10 @ 121° C. decreases of 25-40 % are typical upon completedecarboxylation.

Low bulk viscosity perfluoroelastomer compositions may also be preparedby blending appropriate amounts of the decarboxylated or partiallydecarboxylated perfluoroelastomer compositions of the invention with asecond perfluoroelastomer. The second perfluoroelastomer may be aperfluoroelastomer having ionized or ionizable endgroups, or it may be aperfluoroelastomer having bromine-containing groups or iodine-containinggroups. The resulting perfluoroelastomer blend compositions will havebulk viscosity intermediate between that of the pure perfluoroelastomercomponents. The blends will exhibit physical properties typical of theperfluoroelastomer components, but they will be characterized byenhanced processability, for example extrusion behavior and mixingproperties.

The carbonyl content of the perfluoroelastomers of the invention may bedetermined by an integrated absorbance ratio method based on Fouriertransform infrared analysis. Specifically, the total content ofcarboxyl, carboxylate, and carboxamide groups in the polymer isdetermined by measuring the integrated carbonyl absorbance (i.e. thetotal area of all peaks in the region 1620-1840 cm⁻¹) of thin polymerfilms using a Fourier transform IR spectrometer. In order to compare thecarbonyl level in different polymer samples, integrated absorbance isnormalized for differences in polymer film thickness by taking the ratioof the carbonyl integrated absorbance to the thickness band integratedabsorbance. Thickness band integrated absorbance is the total area ofall peaks in the region 2200-2740 cm⁻¹. The integrated absorbance ofpeaks in the latter region is proportional to the thickness of thepolymer film. The integrated absorbance ratio can be readily used tocalculate the concentration of carbonyl groups in the polymer bycomparing the integrated absorbance ratio of the polymer to that of astandard polymer of known carboxyl or carboxylate content. Suchstandards may be prepared from polymers of this invention which havebeen heated in order to completely decarboxylate them, as described inco-pending U.S. patent application Ser. No. 08/755,919. In the casewhere a carbonyl-containing cure site monomer is present in the polymerchain, the integrated absorbance due to the carbonyl group is subtractedfrom the total integrated absorbance in order to determine theconcentration of the carbonyl-containing endgroups. Known amounts of acarbonyl-containing compound such as ammonium perfluorooctanoate maythen be added to the substantially completely decarboxylated polymer inorder to produce a calibration curve of integrated absorbance ratioversus concentration of ammonium perfluorooctanoate.

Perfluoroelastomer compositions of this invention also comprisecompositions in which polymer plus curing agent is present. Generally,when used commercially, perfluoroelastomer compositions will be composedof a polymeric component, a curing agent, and optional additives. Thepolymeric component is a perfluoroelastomer of the types describedabove.

When the perfluoroelastomer has copolymerized units of anitrile-containing cure site monomer, a cure system based on anorganotin compound can be utilized. Suitable organotin compounds includeallyl-, propargyl-, triphenyl- and allenyl tin curatives. Tetraalkyltincompounds or tetraaryltin compounds are the preferred curing agents foruse in conjunction with nitrile-substituted cure sites. The amount ofcuring agent employed will necessarily depend on the degree ofcrosslinking desired in the final product as well as the type andconcentration of reactive moieties in the perfluoroelastomer. Ingeneral, about 0.5-10 phr of curing agent can be used, and 1-4 phr issatisfactory for most purposes. It is believed that the nitrile groupstrimerize to form s-triazine rings in the presence of curing agents suchas organotin, thereby crosslinking the perfluoroelastomer. Thecrosslinks are thermally stable, even at temperatures of 275° C. andabove. It has been found that the decarboxylated orpartially-decarboxylated perfluoroelastomers have an unacceptably slowcure rate when compounded in accordance with conventional organotincurative recipes unless an accelerator is added. In particular, it hasbeen found that organic or inorganic ammonium salts are unusuallyeffective accelerators. Preferred accelerators include ammoniumperfluorooctanoate, ammonium perfluoroacetate, ammonium thiocyanate, andammonium sulfamate. Ammonium perfluorooctanoate is most preferred. Theseaccelerators are disclosed in U.S. Pat. No. 5,677,389 and are generallyused in quantities of 0.1-2.0 parts per hundred partsperfluoroelastomer, preferably in quantities of 0.5-1.0 parts perhundred parts perfluoroelastomer. In addition, ammonium salts of organicand inorganic acids may be used as curing agents. Suitable ammoniumsalts and quantities effective for curing perfluoroelastomers aredisclosed in U.S. Pat. No. 5,565,512. Fast-curing perfluoroelastomercompositions wherein the perfluoroelastomer component has a plurality ofcarbonyl-containing functional groups and the curative is an organotincurative are disclosed in co-pending U.S. patent application Ser. No.08/755,919.

A preferred cure system, useful for perfluoroelastomers containingnitrile-containing cure sites, utilizes bis(aminophenols) andbis(aminothiophenols) of the formulas ##STR1## and tetraamines of theformula ##STR2## where A is SO₂, O, CO, alkyl of 1-6 carbon atoms,perfluoroalkyl of 1-10 carbon atoms, or a carbon-carbon bond linking thetwo aromatic rings. The amino and hydroxyl groups in formulas XII andXIII above are interchangeably in the meta and para positions withrespect to the group A. Preferably, the curing agent is a compoundselected from the group consisting of 4,4'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene!bis(2-aminophenol);4,4'-sulfonylbis(2-aminophenol); 3,3'-diaminobenzidine; and3,3',4,4'-tetraaminobenzophenone. The first of these is the mostpreferred and will be referred to as bis(aminophenol) AF. The curingagents can be prepared as disclosed in U.S. Pat. No. 3,332,907 toAngelo. Bis(aminophenol) AF can be prepared by nitration of 4,4'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene!-bisphenol (i.e. bisphenolAF), preferably with potassium nitrate and trifluoroacetic acid,followed by catalytic hydrogenation, preferably with ethanol as asolvent and a catalytic amount of palladium on carbon as catalyst. Thelevel of curing agent should be chosen to optimize the desiredproperties of the vulcanizate. In general, a slight excess of curingagent over the amount required to react with all the cure sites presentin the polymer is used. Typically, 0.5-5.0 parts by weight of thecurative per 100 parts of polymer is required. The preferred range is1.0-2.0 parts.

Peroxides may also be utilized as curing agents. Useful peroxides arethose which generate free radicals at curing temperatures. A dialkylperoxide or a bis(dialkyl peroxide) which decomposes at a temperatureabove 50° C. is especially preferred. In many cases it is preferred touse a ditertiarybutyl peroxide having a tertiary carbon atom attached toperoxy oxygen. Among the most useful peroxides of this type are2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexyne-3 and2,5-dimethyl-2,5-di(tertiarybutylperoxy)-hexane. Other peroxides can beselected from such compounds as dicumyl peroxide, dibenzoyl peroxide,tertiarybutyl perbenzoate, and di1,3-dimethyl-3-(t-butylperoxy)butyl!carbonate. Generally, about 1-3parts of peroxide per 100 parts of perfluoroelastomer is used. Anothermaterial which is usually blended with the composition as a part of theperoxide curative system is a coagent composed of a polyunsaturatedcompound which is capable of cooperating with the peroxide to provide auseful cure. These coagents can be added in an amount equal to 0.1 and10 parts per hundred parts perfluoroelastomer, preferably between 2-5parts per hundred parts perfluoroelastomer. The coagent may be one ormore of the following compounds: triallyl cyanurate; triallylisocyanurate; tri(methylyallyl)isocyanurate;tris(diallylamine)-s-triazine; triallyl phosphite; N,N-diallylacrylamide; hexaallyl phosphoramide; N,N,N',N'-tetraalkyltetraphthalamide; N,N,N',N'-tetraallyl malonamide; trivinylisocyanurate; 2,4,6-trivinyl methyltrisiloxane; andtri(5-norbomene-2-methylene)cyanurate. Particularly useful is triallylisocyanurate.

Depending on the cure site monomers present, it is also possible to usea dual cure system. For example, perfluoroelastomers havingcopolymerized units of nitrile-containing cure site monomers can becured using a curative comprising a mixture of a peroxide in combinationwith an organotin curative and a coagent. Generally, 0.3-5 parts ofperoxide, 0.3-5 parts of coagent, and 0.1-10 parts of organotin curativeare utilized.

Additives, such as carbon black, stabilizers, plasticizers, lubricants,fillers, and processing aids typically utilized in perfluoroelastomercompounding can be incorporated into the compositions of the presentinvention, provided they have adequate stability for the intendedservice conditions. In particular, low temperature performance can beenhanced by incorporation of perfluoropolyethers.

Carbon black fillers are used in elastomers as a means to balancemodulus, tensile strength, elongation, hardness, abrasion resistance,conductivity, and processability of the compositions. Inperfluoroelastomer compositions, small particle size, high surface areacarbon blacks have been the fillers of choice. A grade commonly chosenis SAF carbon black, a highly reinforcing black with typical averageparticle size of about 14 nm and designated N 110 in Group No. 1,according to ASTM D-1765. The particular carbon blacks useful in thecompositions of the present invention are those described in WO95/22575. These particular carbon blacks have average particle sizes ofat least about 100 nm to about 500 nm as determined by ASTM D-3849.Examples include MT blacks (medium thermal black) designated N-991,N-990, N-908, and N-907, and large particle size furnace blacks. MTblacks are preferred. When used, 1-70 phr of large size particle blackis generally sufficient, and this amount does not retard cure time.

In addition, fluoropolymer fillers may also be present in thecomposition. Generally from 1 to 50 parts per hundred perfluoroelastomerof a fluoropolymer filler is used, and preferably at least about 5 partsper hundred parts perfluoroelastomer is present. The fluoropolymerfiller can be any finely divided, easily dispersed plastic fluoropolymerthat is solid at the highest temperature utilized in fabrication andcuring of the perfluoroelastomer composition. By solid, it is meant thatthe fluoroplastic, if partially crystalline, will have a crystallinemelting temperature above the processing temperature(s) of theperfluoroelastomer(s). Such finely divided, easily dispersedfluoroplastics are commonly called micropowders or fluoroadditives.Micropowders are ordinarily partially crystalline polymers.

Micropowders that can be used in the composition of the inventioninclude, but are not limited to, those based on the group of polymersknown as tetrafluoroethylene (TFE) polymers. This group includeshomopolymers of TFE (PTFE) and copolymers of TFE with such smallconcentrations of at least one copolymerizable modifying monomer thatthe resins remain non-melt-fabricable (modified PTFE). The modifyingmonomer can be, for example, hexafluoropropylene (HFP), perfluoro(propylvinyl) ether (PPVE), perfluorobutyl ethylene, chlorotrifluoroethylene,or another monomer that introduces side groups into the polymermolecule. The concentration of such copolymerized modifiers in thepolymer is usually less than 1 mole percent. The PTFE and modified PTFEresins that can be used in this invention include both those derivedfrom suspension polymerization and from emulsion polymerization.

High molecular weight PTFE used in production of micropowder is usuallysubjected to ionizing radiation to reduce molecular weight. Thisfacilitates grinding and enhances friability if the PTFE is produced bythe suspension polymerization process, or suppresses fibrillation andenhances deagglomeration if the PTFE is produced by the emulsionpolymerization process. It is also possible to polymerize TFE directlyto PTFE micropowder by appropriate control of molecular weight in theemulsion polymerization process, such as disclosed by Kuhls et al. inU.S. Pat. No. 3,956,000. Morgan, in U.S. Pat. No. 4,879,362, discloses anon-melt-fabricable, non-fibrillating modified PTFE produced by theemulsion (dispersion) polymerization process. This polymer formsplatelets on shear blending into elastomeric compositions, instead offibrillating.

TFE polymers also include melt-fabricable copolymers of TFE havingsufficient concentrations of copolymerized units of one or more monomersto reduce the melting point significantly below that of PTFE. Suchcopolymers generally have melt viscosity in the range of 0.5-60×10³Pa.s, but viscosities outside this range are known. Perfluoroolefins andperfluoro(alkyl vinyl) ethers are preferred comonomers.Hexafluoropropylene and perfluoro(propyl vinyl) ether are mostpreferred. Melt fabricable TFE copolymers such as FEP(TFE/hexafluoropropylene copolymer) and PFA TFE/perfluoro(propylvinyl)ether copolymer! can be used, provided they satisfy constraints onmelting temperature with respect to perfluoroelastomer processingtemperature. These copolymers can be utilized in powder form as isolatedfrom the polymerization medium, if particle size is acceptable, or theycan be ground to suitable particle size starting with stock of largerdimensions.

The curable compositions of the present invention are useful inproduction of gaskets, tubing, and seals. Such articles are produced bymolding a compounded formulation of the curable composition with variousadditives under pressure, curing the part, and then subjecting it to apost cure cycle. The cured compositions have excellent thermal stabilityand chemical resistance. They are particularly useful in applicationssuch as seals and gaskets for manufacturing semiconductor devices, andin seals for high temperature automotive uses.

The invention is now illustrated by certain embodiments wherein allparts are by weight unless otherwise specified.

EXAMPLES TEST METHODS

Cure Characteristics

Cure characteristics were measured using a Monsanto oscillating diskrheometer (ODR), under conditions corresponding to ASTM D 2084. Thefollowing cure parameters were recorded:

M_(max) : maximum torque level, in units of N.m

M_(min) : minimum torque level, in units of N.m

M_(max) -M_(min) difference between maximum and minimum torque, in unitsof N.m

t_(s) 2: minutes to 2.26 N.m rise above M_(min)

t_(c) 90: minutes to 90% of maximum torque

Stress/strain properties were measured according to ASTM D 412. Thefollowing parameters were recorded:

M₁₀₀ : modulus at 100% elongation in units of MPa

T_(B) : tensile strength at break in units of MPa.

E_(B) : elongation at break in units of %

Compression set of O-ring samples was determined in accordance with ASTMD 395.

Example 1

A perfluoroelastomer terpolymer having copolymerized units ofapproximately 54.8 wt. % tetrafluoroethylene (TFE), 43 wt. %perfluoro(methyl vinyl) ether (PMVE) and 2.2 wt. %perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) (8-CNVE), having aninherent viscosity of 0.53 (as measured by dissolving 0.2 g of polymerin 100 ml of a mixed solvent of 60 wt. % FC-437, 40 wt. % FC-75, 3 wt. %diglyme at 30° C.), and an integrated absorbance ratio of 0.46 was usedin this Example.

The perfluoroelastomer terpolymer was prepared by aqueous emulsionpolymerization at 85° C. and approximately 600 psi (4.1 MPa) in acontinuous reactor with agitation using the general preparative methodsdescribed in U.S. Pat. No. 4,281,092. The surfactant for thepolymerization was ammonium perfluorooctanoate and the sole initiatorwas ammonium persulfate. TFE and PMVE monomers were fed from compressorsand liquid cure site monomer, 8-CNVE, was fed neat from a high pressuremetering pump. A buffering salt, disodium hydrogen phosphate, waspresent to control the pH in the range 4.5-6.5 to counteract aciditygenerated by persulfate decomposition. Upon exiting the reactor, thepolymer emulsion was coagulated with an aqueous solution of magnesiumsulfate. The coagulated polymer was then washed with deionized water anddried in a circulating air oven at 80° C. for 48 hours to form a polymercrumb, hereinafter referred to as "Polymer Crumb A". Polymer Crumb A hadMooney Viscosity (ML-10 @ 121° C.) of 116.

Polymer Crumb A was heat treated in a circulating air oven at 300° C.for 60 minutes in order to produce a substantially decarboxylatedpolymer of this invention. This heat-treated polymer had MooneyViscosity (ML-10 @ 121° C.) of 69, which represents a substantialdecrease compared to the Mooney viscosity of Polymer Crumb A. As aresult of its lower viscosity, the heat treated polymer of thisinvention is easier to process than the non-heat treated Polymer CrumbA.

The heat treated polymer was then compounded with 30 phr medium thermalcarbon black (MT black) and 1.0 phr2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane i.e. bis(aminophenol)AF! curative on a rubber mill to produce a curable composition of thisinvention. Cure response was determined by ODR, according to ASTM D2084. Results are shown in Table I. Stress/strain properties weremeasured on one set of test specimens which had been died out of polymersheet press-cured at 210° C. for 20 minutes and on a second set whichhad been press-cured, then post-cured at 300° C. for 18 hours in an airoven. Values of compression set for O-ring samples press-cured at 275°C. for 20 minutes and for O-rings press-cured, then post-cured for 18hours at 300° C. in an air oven also are shown in Table I.

Comparative Example A

A sample of Polymer Crumb A which had not been subjected to heattreatment was compounded with carbon black and curative as described inExample 1. Cure response was determined by ODR according to ASTM D 2084.Results are shown in Table I.

It can be seen from the data shown in Table I that the curablecomposition consisting of heat treated, substantially decarboxylatedpolymer of Example 1 exhibited a faster cure response, a higher curestate, and a lower minimum viscosity than the non-heat treatedcomparative sample, as indicated by the lower tc90, higher M_(max)-M_(min), and lower M_(min) of the Example 1 composition.

                  TABLE I    ______________________________________                  Example 1                         Comparative Ex. 1    ______________________________________    Physical Properties    ODR    M.sub.max (N.m) 4.3      4.1    M.sub.min (N.m) 0.3      0.7    M.sub.max -M.sub.min (N.m)                    4.0      3.4    t.sub.s 2 (minutes)                    4.2      4.5    t.sub.c 90 (minutes)                    7.8      12.0    Stress/Strain Properties    Press-cured    M.sub.100 (MPa) 12.7     --    T.sub.B (MPa)   15.9     --    E.sub.B (%)     187      --    Post-cured    M.sub.100 (MPa) 14.8     --    T.sub.B (MPa)   23.2     --    E.sub.B (%)     173      --    Compression Set    Press-cured (%) 37       --    Post-cured (%)  20       --    ______________________________________

Example 2

Polymer Crumb B was prepared in substantially the same manner asdescribed for preparation of Polymer Crumb A in Example 1 except aslightly higher R_(i) /R_(p) was used. Polymer Crumb B had MooneyViscosity (ML-10 @ 121° C.) of 85, an inherent viscosity of 0.48, and anabsorbance ratio of 0.430. The Polymer Crumb B was formed into avoid-free 4 mm thick sheet by pressing at 150° C. for 3 minutes. Thesheet was then heat-treated under various conditions in a circulatingair oven in order to partially decarboxylate the polymer. Heat treatmentof this polymer at 300° C. for one hour resulted in reduction of Mooneyviscosity, ML-10 @ 121° C., to 58.

Fourier Transform infrared spectra were run on thin films pressed at150° C. for 2 minutes to measure the carbonyl-containing endgroupconcentration as a function of the heating time and temperature. Thedata in Table II show the effect of various heat treatments on theintegrated Absorbance Ratio. Absorbance Ratio is also expressed as thefraction of remaining carbonyl-containing endgroups by dividing theAbsorbance Ratio of the indicated sample by that of Polymer Crumb B.

                  TABLE II    ______________________________________                              Fraction Carboxyl or    Time/Temperature in Air Oven                    Absorbance                              Carboxylate Endgroups    (4 mm Thick Polymer Sheet)                    Ratio     Remaining    ______________________________________    15 minutes/255° C.                    0.430     1.00    30 minutes/255° C.                    0.371     0.863    60 minutes/255° C.                    0.255     0.592    15 minutes/270° C.                    0.368     0.851    30 minutes/270° C.                    0.197     0.459    60 minutes/270° C.                    0.135     0.314    15 hours/270° C.                     0.0322    0.0750    15 minutes/280° C.                    0.250     0.582    30 minutes/280° C.                    0.186     0.433    60 minutes/280° C.                     0.0695   0.162     5 minutes/300° C.                    0.427     0.994    10 minutes/300° C.                    0.338     0.786    15 minutes/300° C.                    0.211     0.491    30 minutes/300° C.                    0.147     0.341    60 minutes/300° C.                     0.0805   0.187    ______________________________________

The rheological properties of several samples that were heat-treated at300° C. (Table II) were evaluated using a Rosand Capillary Rheometer ata shear rate of 1500/sec, at 90° C. Samples were cut from the 4 mmthick, void-free sheets described above. The results of that evaluation,as shown in Table III, indicate that both the shear stress and the shearviscosity are sharply reduced by heat treating the polymer. The datafurther indicate that this effect is dependent on heating time (TableIII).

                  TABLE III    ______________________________________    Time in 300° C. Air Oven    (Minutes)      Shear Stress (kPa)                               Shear Viscosity (Pa.s)    ______________________________________     0             2151        1434     5             1200        800    10              705        470    15              606        404    ______________________________________

Example 3

A perfluoroelastomer was prepared in substantially the same manner asthe perfluoroelastomer of Example 1, except that a lower R_(i) /R_(p)was used. After isolation, the polymer crumb was heat treated in acirculating air oven at 300° C. for 60 minutes in order to produce asubstantially decarboxylated polymer of this invention. Mooney Viscosity(ML-10 @ 121° C.) of 106, and an integrated absorbance ratio of 0.32.This substantially decarboxylated polymer is designated Polymer 1 inTable IV. Polymer 1 and non-heat-treated perfluoroelastomer (designatedPolymer 2) were compounded with the additives shown in Table IV. ODRtest specimens were prepared from the compounded samples. As indicatedby the 200° C. ODR data shown in Table IV, curable compositions ofPolymer 1 cured very slowly with tetraphenyltin curative. Samples ofnon-heat treated Polymer 2 cured well with tetraphenyltin. When ammoniumperfluorooctanoate, which is a source of ammonium carboxylate salt, wasadded to Polymer 1, cure was satisfactory, i.e . there was asurprisingly sharp increase in cure rate. In addition, the sampleexhibited an increased cure state.

                  TABLE IV    ______________________________________                                     Example 3C    Component  Example 3A Example 3B (Comparative)    ______________________________________    Polymer 1  100        100        --    Polymer 2  --         --         100    SRF Carbon Black               10         10         10    Tetraphenyltin               2          2          2    Ammonium   --         1          --    Perfluorooctanoate    ODR Cure Response (200° /24 minutes)    M.sub.max (N.m)               1.0        3.1        3.5    M.sub.min (N.m)               0.48       0.70       1.0    M.sub.max -M.sub.min (N.m)               0.52       2.4        2.5    ts2 (minutes)               17.0       1.9        3.8    tc50 (minutes)               18.0       3.1        8.1    tc90 (minutes)               23.0       11.6       18.4    ______________________________________

Example 4

A perfluoroelastomer containing copolymerized units oftetrafluoroethylene, perfluoro(methyl vinyl) ether, andperfluoro-8(cyano-5-methyl-3,6-dioxa-1-octene) in a molar ratio ofapproximately 67.2/32.1/0.7 was prepared as follows: an aqueous solutionconsisting of 20 liters of deionized water, 93 g of ammonium persulfate,810 g of disodium hydrogen phosphate heptahydrate and 182 g of ammoniumperfluorooctanoate (Fluorad® FC-143 fluorinated surfactant) was pumpedinto a 5 liter mechanically stirred, water-jacketed stainless steelautoclave at a rate of 688 ml/hour. A third stream consisting of 22.3g/hour of perfluoro-(8-cyano-5-methyl-3,6-dioxa- 1 -octene) was meteredin simultaneously. By means of a diaphragm compressor a gaseous mixtureof tetrafluoroethylene (363 g/hour) and perfluoro(methyl vinyl)ether(412 g/hour) monomer was fed in at a constant rate. The temperature ofthe reactor was maintained at 85° C. and 6.2 MPa (900 psi) pressurethroughout the reaction and the pH was controlled at 6.6. The polymerlatex was removed continuously by means of a let down valve andunreacted monomers were vented. The latex from 16 hours of operation wascollected and the polymer was isolated as follows: 5 liters of the abovelatex was added with stirring to a preheated (90°-95° C.) solutionconsisting of 225 g of magnesium sulfate heptahydrate and 40 liters ofdeionized water. The coagulated crumb polymer which resulted wasfiltered, washed repeatedly with water, and dried in an air oven at 70°C. for 48 hours. The dried polymer weighed 9489 g and had the followingcomposition, 42.9 w.t % perfluoro(methyl vinyl) ether, 2.2 wt. %perfluoro(8-cyano-5-methyl-3,6-dioxa- 1 -octene), the remainder beingtetrafluoroethylene. The polymer had inherent viscosity of 0.74 dl/gmeasured in a solution containing 0.1 g of polymer per 100 g of solventconsisting of 60/40/3 volume ratio of heptafluoro-2,2,3-trichlorobutane,perfluoro(α-butyltetrahydrofuran) and ethylene glycol dimethyl ether.

The polymer was extruded in a 30 mm twin screw extruder equipped with avacuum port, operating at approximately 30 in Hg vacuum (760 mm Hg), anda single hold die, approximately 0.188 in. in diameter. The temperatureprofile was as shown in Table V.

                  TABLE V    ______________________________________    Extruder Zone  Temperature    ______________________________________    Feed Zone      150    1              315    2              315    3              315    4              315    Die            Unheated    ______________________________________

The polymer melt temperature, as measured with a hand-held pyrometer was320° C. The polymer was fed to the extruder at the rate of 4.54 kg/hourusing a weigh feeder. The screw speed was 100 rpm. The IntegratedAbsorbance Ratio, Mooney viscosity, and melt rheology were measured.Results are shown in Table VI.

                  TABLE VI    ______________________________________    Properties      Before Extrusion                                After Extrusion    ______________________________________    Absorbance Ratio                    0.39        0.21    Mooney Viscosity                    18          17    Melt Rheology, Rosand    Capillary @ 90° C.    Shear Viscosity (Pa.s)    Shear Rate 600/s                    1014        779    Shear Rate 1500/s                    697         322    ______________________________________

Example 5

a perfluoroelastomer terpolymer, 5A, was prepared by aqueous emulsionpolymerization at 85° C. and approximately 600 psi (4.1 MPa) in acontinuous reactor with agitation using the general preparative methodsdescribed in U.S. Pat. No. 4,281,092. The surfactant for thepolymerization was ammonium perfluorooctanoate and the sole initiatorwas ammonium persulfate. TFE and PMVE monomers were fed from compressorsand liquid cure site monomer, 8-CNVE, was fed neat from a high pressuremetering pump. A buffering salt, disodium hydrogen phosphate, waspresent to control the pH in the range 4.5-6.5 to counteract aciditygenerated by persulfate decomposition. Upon exiting the reactor, thepolymer emulsion was coagulated with an aqueous solution of magnesiumsulfate. The coagulated polymer was then washed with deionized water andwas dried at 80° for 48 hours. Perfluoroelastomer terpolymer 5A had acomposition of approximately 56 wt. % tetrafluoroethylene (TFE), 42 wt.% perfluoro(methyl vinyl) ether (PMVE) and 2 wt. %perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) (8-CNVE), a MooneyViscosity (ML-10 @ 121° C.) of 82.5, and an integrated absorbance ratioof 0.232. Perfluoroelastomer 5A was heat-treated in a circulating airoven at 235° C. for 15 minutes. The heat-treated polymer had MooneyViscosity (ML-10 @ 121° C.) of 82.2 and an integrated absorbance ratioof 0.212.

A second perfluoroelastomer terpolymer of the invention, 5B, wasprepared in a substantially the same manner as perfluoroelastomer 5A,except that potassium persulfate was used as the initiator, potassiummonohydrogen phosphate was used as a buffer, and potassiumperfluorooctanoate was used as a surfactant. After isolation and dryingat 80° for 48 hours, perfluoroelastomer terpolymer 5B had a compositionof approximately 56 wt. % tetrafluoroethylene (TFE), 42 wt. %perfluoro(methyl vinyl) ether (PMVE) and 2 wt. %perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) (8-CNVE), a MooneyViscosity (ML-10 @ 121° C.) of 108.2 and an integrated absorbance ratioof 0.232. Perfluoroelastomer 5B was heat-treated in a circulating airoven at 235° C. for 15 minutes. The heat-treated polymer had MooneyViscosity (ML-10 @ 121° C.) of 102.1 and an integrated absorbance ratioof 0.070.

A blend of 50 parts by weight of isolated and dried perfluoroelastomer5A and 50 parts by weight isolated and dried perfluoroelastomer 5B wasprepared. The blend had a Mooney Viscosity (ML-10 @ 121° C.) of 116.3and an integrated absorbance ratio of 0.190. The blend was heat-treatedat 240° C. for 15 minutes. The heat-treated polymer had Mooney Viscosity(ML-10 @ 121° C.) of 105.6 and an integrated absorbance ratio of 0.099.

I claim:
 1. A process for preparation of an uncured perfluoroelastomercomposition comprising the steps ofA) preparing a perfluoroelastomerhaving a plurality of carbonyl-containing functional groups selectedfrom the group consisting of carboxyl endgroups, carboxylate endgroups,carboxamide endgroups, and mixtures thereof by copolymerizing a monomermixture comprising a) a perfluoroolefin monomer, b) a perfluorovinylether monomer selected from the group consisting of perfluoro(alkylvinyl) ethers, perfluoro(alkoxy vinyl) ethers, and mixtures thereof, andc) a cure site monomer at a pressure of from 4-10 MPa, in the presenceof a persulfate free radical initiator in a polymerization mixturewherein i) the feed ratio of monomer to initiator is controlled so thatthe ratio of the radical flux to the polymerization rate, R_(i) /R_(p),is from about 10 to 50 millimoles per kilogram, and ii) less than 5 molepercent of a sulfite or bisulfite reducing agent, based on the totalmoles of persulfate initiator and reducing agent, is present in thepolymerization mixture; B) isolating said perfluoroelastomer having aplurality of carbonyl-containing functional groups; and C) heating saidisolated perfluoroelastomer having a plurality of carbonyl-containingfunctional groups at a temperature of at least 230° C. for a timesufficient to at least partially decarboxylate the perfluoroelastomer.2. A process of claim 1 wherein the heating in step C) takes place at atemperature within the range of 280° C.-320° C.
 3. A process of claim 1wherein step C) is accomplished by heating in an oven.
 4. A process ofclaim 1 wherein step C) is accomplished by heating in an extruder.
 5. Aprocess of claim 1 wherein step C) is accomplished by heating in acompression mold.
 6. A process of claim 1 wherein step C) isaccomplished by heating using microwave radiation.
 7. A process of claim1 wherein the persulfate free radical initiator is potassium persulfate.8. A process of claim 1 wherein the persulfate free radical initiator isammonium persulfate.